But that means that they'll have to install gdebi and its dependencies first without a Net connection, anyway!

Even after that, gdebi won't pull in any missing dependencies without a Net connection, but I don't think that kernel has any, so "dpkg -i <deb>" will probably work. If not, I know the Liquorix kernel linux-image package will.

I would double check first just to make sure that firmware-iwlwifi is installed,though.

as sysadmin and looking for your PCI ID [8086:24fd] in the list of supported devices, but the output might look like a lot of technobabble to a beginner. Can you find yours now in the output? It's there, and should be missing for the 4.9 kernel.